The conventional fixing mechanism for fastening to a vertebral fixation rod is generally provided with a screw, which is used to fasten a vertebra intended to be fixed and which is also used to urge forcibly the vertebra to remain on the vertebral fixation rod and the like, as exemplified by the U.S. Pat. Nos. 5,261,913; 4,887,596; and 5,257,993. Such fastening methods as described above are defective in design in that a slight intervertebral motion can often cause the fastening screw of the fixing mechanism to become loosened or even detached.